Complex compounds which contain, as the central atom, a metal of Group VIII of the Periodic Table of the Elements (IUPAC Version) and, as ligands, P(III) compounds such as phosphines and phosphites, as well as optionally other groups suited for complex formation, have recently become increasingly important as catalysts. Thus, the reaction of olefins with synthesis gas to give aldehydes (hydroformylation), practiced industrially on a large scale, is carried out in the presence of catalyst systems which are composed of cobalt and, in particular, rhodium, and triphenylphosphine. Catalysts based on complex compounds containing phosphines have also proved suitable for the reaction of methanol with synthesis gas to give higher alcohols, in particular ethanol and propanol (homologization). In accordance with the solubility of these catalysts in organic media, the reactions are carried out in the homogeneous phase.
Instead of in the homogeneous phase, the reaction can also be carried out in heterogeneous reaction systems. The advantage of this process variant is the simple and gentle separation of the catalyst, which is dissolved in water, from the water-insoluble reaction product. For example, the process described in DE 27 00 904 C2 for the addition of hydrogen cyanide to an unsaturated organic compound having at least one ethylenic double bond works according to this principle. Suitable catalysts for this reaction are the systems nickel/TPPTS [TPPTS is tri(sulfophenyl)phosphine], palladium/TPPTS, or iron/TPPTS. For the preparation of aldehydes by reaction of olefins with carbon monoxide and hydrogen, according to the process of DE 26 27 354 C2, rhodium is employed as metal or in the form of one of its compounds, together with a water-soluble phosphine, for example TPPTS, as the catalyst.
Diphosphines, which as bidentate ligands are able to form chelates with metal ions, are used only rarely, in contrast to the monophosphines, and then exclusively as constituents of homogeneously dissolved catalysts. Thus, according to the teaching of DE 29 04 782 C2, aldehydes are obtained by hydroformylation of a lower olefinic compound in an organic solvent in the presence of a rhodium complex, a trisubstituted monophosphine, and a diphosphinoalkane.
DE 29 09 041 A1 describes a process for the preparation of aldehydes by hydroformylation of olefins in which platinum is present as the catalyst, the halide of at least one metal of Group IVB of the Periodic Table ("carbon group") is present as the auxiliary catalyst. A two-bonded ligand of the formula R.sub.2 X-Z-Y-Z-XR'.sub.2 (R and R' are each an alkyl, aryl, or aralkyl group; X is phosphorus, arsenic, or antimony; Y is alkylene, arylene, or aralkylene; and Z is methylene or oxygen) is present as the reaction promoter. 2,2'-Bis(diphenylphosphinomethyl)-1,1'-binaphthyl is used in combination with a rhodium or nickel compound as a ligand for asymmetric hydrogenation catalysts according to Laid-open Japanese Patent Application 79/39,059.
A reason for the comparatively rare use of diphosphines as a constituent of catalysts may be the difficulties which stand in the way of their preparation on an industrial scale. A number of laboratory processes for obtaining diphosphines are indeed known, bu their application to industrial production processes is not without problems, both technical and economic.
A process which, inter alia, relates to the preparation of diphosphines - they are used as bidentate phosphorus ligands - is the subject of EP 326,286 Al. Biaryl compounds are employed as starting substances which are substituted in each of the two aryl groups by the radical --CH(R.sub.3)(R.sub.4) and optionally by other radicals. They are converted by the action of proton-eliminating reagents into biaryldianions, which are reacted with phosphorus compounds of the formula X--P(R.sub.1)(R.sub.2) or X--PO(R.sub.1)(R.sub.2), X being preferably a halogen atom. In this manner, diphosphines are obtained directly or, if the phosphorus compound X--PO(R.sub.1)(R.sub.2) was employed, after reduction.
The process described above is suitable only for the preparation of diphosphines which are not substituted or contain substituents which are inert to compounds having reducing action. In this connection, it must be recognized that a reduction step is not only necessary when using reactants of the type X--P(O)(R.sub.1)(R.sub.2). The formation of the biaryldianion also takes place under reducing conditions, as the reagents employed for eliminating the proton, (such as alkali metal hydrides or alkali metal alkyls) have a reducing action. A direct preparation of biarylphosphines containing sulfonic acid groups is, therefore, not possible by the methods described above, because the sulfonic acid groups are not retained in the reaction of the biaryl and phosphorus compounds.
Therefore, the object was to develop a process for the preparation of sulfonated diphosphines which not only solves the problems described, but is also simple to carry out industrially and, moreover, is economical.